The subject invention relates to the processing and vulcanization of diene polymer and copolymer elastomer containing rubber stocks. More specifically, the present invention relates to the processing and vulcanization of diene polymer and copolymer elastomer-containing, silica-filled rubber stocks using a fatty acid ester of a polyol as a processing aid.
In another embodiment, the present invention relates to the processing and vulcanization of diene polymer and copolymer elastomer-containing, silica-filled rubber stocks containing additional mineral fillers.
In the art it is desirable to produce elastomeric compounds exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents, followed by vulcanization. Such elastomers, when compounded, fabricated and vulcanized into components for constructing articles such as tires, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use.
The hysteresis of an elastomer refers to the difference between the energy applied to deform an article made from the elastomer and the energy released as the elastomer returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn, result in lowered fuel consumption of vehicles using such tires.
In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomeric properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like) and a vulcanizing system such as sulfur-containing vulcanizing (that is, curing) system.
Previous attempts at preparing readily processable, vulcanizable, silica-filled rubber stocks containing natural rubber or diene polymer and copolymer elastomers have focused upon the sequence of adding ingredients during mixing (Bomal, et al., Influence of Mixing procedures on the Properties of a Silica Reinforced Agricultural Tire Tread, May 1992), the addition of de-agglomeration agents such as zinc methacrylate and zinc octoate, or SBR-silica coupling agents such as mercapto propyl trimethoxy silane (Hewitt, Processing Technology of Silica Reinforced SBR, Elastomerics, pp 33-37, March 1981), and the use of bis[3-(triethoxysilyl)propyl]tetrasulfide (Si69) processing aid (Degussa, PPG).
The use of Si69 processing aid in the formulation of silica-filled rubber stocks has been successful, but generally requires a large amount of the additive, such as 10% by weight based on the weight of silica, in order to be effective.
Precipitated silica has been increasingly used as a reinforcing particulate filler in carbon black-filled rubber components of tires and mechanical goods. Silica-loaded rubber stocks, however, exhibit relatively poor processability.
The present invention provides a fatty acid ester of a polyol for use as a processing aid for silica-filled rubber stocks, which greatly improve the processability and properties of the formulations and the resulting vulcanized product. In another embodiment, the present invention further provides additional mineral fillers for use in silica-filled elastomeric rubber stocks, improving tear strength and lowering hysteresis.
It is therefore an object of the present invention to provide rubber stock processing aids which improve the processability of formulations of diene polymer elastomers with silica-filler.
It is another object of the present invention to provide a method for reducing the viscosity of silica-filled elastomeric vulcanizable compounds.
It is another object of the present invention to provide a method for enhancing the processability of silica-filled elastomeric vulcanizable compounds.
It is another object of the present invention to provide vulcanizable silica-filled elastomeric compounds having enhanced physical properties, including decreased hysteresis and increased tear strength.
The foregoing objects, together with the advantages thereof over the existing art, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The present invention provides a process for the preparation of a silica-filled, vulcanized elastomeric compound comprising mixing an elastomer with an amorphous silica filler, a processing aid comprising a fatty acid ester of a polyol, and a cure agent; and, effecting vulcanization. Preferably, the elastomer is a diene monomer homopolymer or a copolymer of a diene monomer and a monovinyl aromatic monomer, and the processing aid is sorbitan monooleate.
The present invention further provides a vulcanizable silica-filled compound comprising an elastomer, a silica filler, a processing aid comprising a fatty acid ester of a polyol, and a cure agent. Preferably, the elastomer is styrene butadiene rubber, and optionally the vulcanizable compound contains natural rubber. The compound is more readily processable during mixing, due to the use of the mixture of the inventive processing aids.
In another embodiment, the present invention provides a vulcanizable silica-filled compound comprising an elastomer, a silica filler, an additional mineral filler such as talc, mica or clay, and a cure agent. Preferably, the vulcanizable compound contains a processing aid comprising a fatty acid of a polyol, such as sorbitan oleate.
The present invention further provides a pneumatic tire comprising tread stock vulcanized from the inventive vulcanizable silica-filled compound.